The Environmental Protection Agency (EPA) has estimated that the annual generation of hazardous wastes is in excess of seventy billion gallons. Hazardous wastes include organic materials, such as polychlorinated biphenyls, pesticides, herbicides, municipal garbage,hospital wastes including pathogens, paints, inks and contaminated solvents, black liquor and explosives. Examples of inorganic wastes include oxides of iron, zinc, copper, lead, magnesium, aluminum, chromium and cadmium,various powdered metal manufacturing residues and metal-containing sludges.
Further, the EPA has classified as toxic, ignitable, corrosive or dangerously reactive many common waste materials, such as paint sludge from appliance factories, dregs of chrome and nickel from metal plating shops, spent raw materials for varnish, carpets and detergents at chemical plants, solvents from dry-cleaned clothes and degreased microchips from computers, mercury from exhausted watch batteries, butane residue from disposable cigarette lighters and lye from cans of oven cleaners.
Landfills are becoming less available as a means of disposing of such wastes. In the absence of suitable landfills, hazardous wastes must be transformed to benign and, preferably, useful development of alternative methods of treating hazardous wastes. Various types of reactors which have been employed for decemposition of hazardous wastes include, for example, liquid injection, multiple hearth, multiple chamber, fluidized bed, molten salt and high efficiency-type boiler reactors. However, many reactors release gases which must be contained or destroyed. Often, these gases are burned, which generally causes formation of free radicals because of the short residence time of the off-gases at flame temperature.
Rotary kilns are a commonly-used type of reactor for combustion of organic wastes. Combustion in rotary kilns typically is initiated by a high temperature flame, whereby reactive species are generated from the organic wastes and then oxidized. However, it is often difficult to contact reactive species with oxygen for oxidation because of poor mixing within the rotary kilns. The rate of destruction of the waste, therefore, can be impeded. Moreover, heat released by combustion occurs away from the flame tip as reacting materials are fed through the rotary kiln, thereby limiting the heat which can be utilized for the initial step of generating reactive species.
Titus, et al., U.S. Pat. No. 3,812,620, disclose a molten pool of glass and miscellaneous metals formed during incineration of "heterogenous waste materials," such as municipal garbage. Various organics in the waste materials are decomposed in the molten pool at temperatures of up to 10,000.degree. F. However, the decomposed materials often are not contacted with oxygen introduced into the molten pool.
Molten salt processes are also known, and are typified by the consumption of solvents and salts in a caustic molten bath into which oxygen and wastes are injected. However, such baths are generally limited to degradation of wastes by chemical reaction rather than thermal destruction. See, for example, U.S. Pat. Nos. 4,447,262, 4,246,255 and 4,017,271.
Oxidation of wastes by reaction of organic materials and oxygen in the presence of water at high temperature and pressure is another method of disposal. See, Wilhelmi, A. R. et al., Chem. Eng. Prog., 75:46-52 (1979). However, inorganic salts, char and metal-containing solids can accumulate at the walls of apparatus employed for such methods, thus necessitating removal of accumulated deposits, causing corrosion and limiting the operating capacity and useful life of such apparatus.